An effect of a drug which is not that which the manufacturer, prescriber and patient require. Some side-effects are almost inevitable: for example, drowsiness with older ANTIHISTAMINE DRUGS; others are very rare, such as REYE'S SYNDROME with ASPIRIN. Some can be predicted to occur if the dose is raised, while others may occur at the lowest of doses due to the individual patient's susceptibility. In deciding whether to prescribe or take a drug, a balance must always be struck between its benefits and risks. (See also MEDICINES – Safe use of medicines.)